Reactions between biological molecules exhibit an extremely high degree of specificity. It is this specificity that provides a living cell with the ability to carry out thousands of chemical reactions simultaneously in the same "vessel". In general, this specificity arises from the "fit" between two molecules having very complex surface topologies. For example, an antibody binds a molecule displaying an antigen on its surface because the antibody contains a pocket whose shape is the complement of a protruding area on the antigen. This type of specific binding between two molecules forms the basis of numerous biological assays.
For example, nucleic acids are linear polymers in which the linked monomers are chosen from a class of 4 possible sub-units. In addition to being capable of being linked together to form the polymers in question, each unit has a complementary sub-unit to which it can bind electrostatically. In the case of DNA, the polymers are constructed from four bases that are usually denoted by A, T ,G, and C. The bases A and T are complementary to one another, and the bases G and C are complementary to one another. Consider two polymers that are aligned with one another. If the sequences in the polymers are such that an A in one chain is always matched to a T in the other chain and a C in one chain is always matched to a G in the other chain, then the two chains will be bound together by the electrostatic forces. Hence, an immobilized chain can be used to bind the complementary chain. This observation forms the basis of tests that detect the presence of DNA or RNA that is complementary to a known DNA or RNA chain. Such detection forms the basis of a number of medical and/or diagnostic tests.
The methods by which the binding of the mobile reactant to the immobilized component of the system is measured varies with the particular reactants. However, a significant fraction of all of the tests involve the measurement of a fluorescent dye that is associated with either the bound or mobile reactant. The dye may be attached to the reactant from the beginning of the process or it may be added through various chemical steps after the mobile and immobilized reactants have been brought into contact with one another.
Systems for medical diagnosis often involve a bank of tests in which each test involves the measurement of the binding of one mobile component to a corresponding immobilized component. To provide inexpensive test kits, systems involving a matrix of immobilized spots have been suggested. Each spot includes the immobilized component of a two component test such as described above. The fluid to be tested is typically brought into contact with the matrix. After chemical processing, the amount of fluorescence associated with each of the spots in the matrix is measured.
The matrix is typically constructed by dispensing small quantities of the immobilized component onto a substrate such as glass that has been chemically modified to bind the immobilized component. The amount of material in each spot is relatively small; however, the number of spots may be quite large. Hence, the generation of such an assay plate requires a reliable microdispenser that can place the individual spots at predetermined locations with a high degree of precision.
The dispenser must also operate without clogging over a large number of samples. In addition, the dispenser must be able to change samples quickly, as each spot in the matrix requires a different immobilized component.
Broadly, it is the object of the present invention to provide an improved dispensing system and method for fabricating assay plates having a matrix of test spots thereon.
It is a further object of the present invention to provide an assay plate dispensing apparatus that is with improved immunity to clogging.
It is a still further object of the present invention to provide a method of fabricating assay plates that is adapted to the large number of different immobilized species that must be applied to each assay plate.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.